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xv                              CASTOREI3ST   AND   ITS   USES                          469

forefathers. The Rev. Edward Topsell observed that " for giving
great ease unto the gowt the skinnes of beavers burned with drie
oynions " are excellent. Castorein as a drug, if not in actual use,
has quite recently been a part of the pharmacopoeia. It is
derived from the anal glands common to this and other Kodents,
and indeed many other mammals.

A large extinct form of Beaver is Trogontherium,1 found in the
" Forest-bed " of Cromer. The skull is about one-fourth longer
than that of Oast or. It has a less inflated bulla, and slightly more
pronounced postorbital processes than Castor. The third molar
(fourth grinding tooth) is relatively larger than in Castor, and has
a rather more folded crown. The foramen magnum is more
triangular.

Fam. 4. Hapiodontidae.A separate family seems to be re-
quired for the genus Haplodon, whose characters will therefore be
merged with those of its family. It is to be distinguished from
most other Squirrel-like creatures by the fact that there is no post-
orbital process to the frontal. The molar teeth are five in the
upper and four in the lower jaw. The Sewellel, J5T. ritfus, like
the other species of the genus (JJ. mcgor\ is found in North
America west of the Hocky Mountains. It has the habit of the
Prairie-marmot, and has a short tail, only moderately long ears,
and five-toed feet. Tullberg is of opinion that this animal nearly
represents the ancestral form of the Squirrel tribe.

SECTION  2.    MYOMORPHA.
This subdivision of the Hodents contains, according to Mr.
Thomas's recent estimate/ no less than 102 genera. It is there-
fore obviously impossible to do more than refer to some of the
more interesting of these. This group is again divided into the
following families:
(1)  Grliridae, including the Dormice.
(2)  Muridae, the Itats, Mice, Gerbiiles, Australian Water-rats,
Hamster.
(3)  Bathyergidae, Cape Mole, etc.
(4)  Spalacidae, IBamboo Hats.
1 E. T. Newton, Trans, Zool, Soe. xlii. 1892, p. 165.
* JVoc. Zool. Soc* 1896, p. 1O16.